I have to say I read about 1/3, glance through 1/3, and skip 1/3. Although I have to include that I read a decent amount of non-fiction career related material, and some of the intros to subjects like Metallurgy, and Organizational Behavior may not really be all that interesting.
I remember him giving a bit to much info in the intros to the stories in Everything's Eventual. Classic novels often have an intro by an academic and I always read these after I've read the novel because they almost always spoil something.
Stephen's after words... I usually wait til the next day to read them because I feel they distract from the closure of the novel.
With King I always read the intros, afterwards and whatever else there is printed on the pages. When I was in my teens, I wouldn't bother with any of that stuff, until I started reading SK. By the way- there is a really rather enlightening afterword in the original paperback editions of Firestarter that wasn't in the hardcover version. Very interesting, indeed!
They're usually the first thing I look for when I get one of his books. I also enjoy it when he quotes song lyrics and poetry before chapters or sections. I've discovered Yeats, T.S. Eliot and Theodore Roethke from reading those.
I always read introductions whether they are by the author who wrote a particular book or by a different author. I actually prefer introductions by a different author than the one who wrote the book most of the time. But on the whole I just read introductions in general because I've always believed in reading a book from first page to last even if the info contained within the introduction is a bit trite.
But SK's introductions are generally pretty interesting.
I don't really like afterwords, though. At least not until after I've had enough time to digest the contents of the novel or story collection. Which, depending on the book, could be a while.